Gas turbine components and steam turbine boiler components are rotary engines that may extract energy from a fluid or gas stream for purposes such as power generation. Such components may operate in relatively high temperature and/or oxidizing environments and may operate in frequent on-off modes depending on the system requirements. Accordingly, the turbine components may experience somewhat constant or regular thermal cycling, which may result in relatively high thermal stresses imparted to the components over the component life time, which may be, in some cases 15 to 30 years.
Coating compositions have been used to protect the turbine components from oxidation, including MCrAl type coatings, wherein M may include a transition metal such as iron, cobalt, nickel or a combination thereof. M may also include other transition metals such as manganese, yttrium, etc. Upon exposure to relatively high temperatures, (700° F. to 1000° F.,) aluminum at the outer surface of the coating may oxidize forming a relatively thin protective Al2O3 oxide layer. Where the MCrAl coating is of nanocrystalline or ultrafine grain structure, Al2O3 formation may be accelerated and in the case of nanocrystalline MCrAl coatings, the Al2O3 may adhere relatively well to the MCrAl coating, resisting spallation. Spallation may be promoted by high thermal stresses due to repetitive thermal cycling, which may then lead to the oxidation of additional aluminum and may result in the consumption of aluminum in the MCrAl coating.
Furthermore, at relatively high temperatures, aluminum may not only be consumed in oxidation but may also diffuse into the substrate, reducing aluminum available for oxidation. It may also be appreciated that while Al2O3 may adhere relatively well to nanocrystalline MCrAl coatings, a nanocrystalline coating structure may accelerate diffusion of Al through the coating and into the substrate or for consumption in oxidation. It is believed that in some cases 80 to 90% of aluminum in nanocrystalline MCrAl may be consumed in a few hundred hours of exposure at elevated temperatures, limiting the durability of the coating and potentially rendering the coating unsuitable for use in turbine applications.